test(apple): CoreGateway seam, XCTest suite, and CI

- Introduce a CoreGateway protocol so feature models depend on a seam
  (CoreRepository conforms); enables faking the core in tests
- Add a VniDropTests target with 42 tests mirroring the KMP suites:
  approval coordinator, send/receive/settings/app models, preferences,
  file previews, invitation decode, message queue, error mapping
- Add a fake gateway/file-system/device-info and fixtures
- Add .github/workflows/apple.yml: build the Rust core, generate the
  project, and run the tests on an iOS Simulator

apple/VniDrop/Features/Receive/ReceiveModel.swift

@@ -51,13 +51,13 @@ final class ReceiveModel: ObservableObject {
 	@Published private(set) var state = ReceiveState()
 	@Published private(set) var coreState = CoreState()
 
-	private let repository: CoreRepository
+	private let repository: CoreGateway
 	private let fileSystemService: FileSystemService
 	private let messages: UiMessageController
 	private var cancellables = Set<AnyCancellable>()
 
 	init(
-		repository: CoreRepository,
+		repository: CoreGateway,
 		fileSystemService: FileSystemService,
 		preferences: AppPreferencesRepository,
 		messages: UiMessageController
@@ -66,7 +66,7 @@ final class ReceiveModel: ObservableObject {
 		self.fileSystemService = fileSystemService
 		self.messages = messages
 
-		repository.$state.sink { [weak self] in self?.coreState = $0 }.store(in: &cancellables)
+		repository.statePublisher.sink { [weak self] in self?.coreState = $0 }.store(in: &cancellables)
 
 		preferences.$preferences
 			.sink { [weak self] prefs in